This invention relates to optical fibers (or optical waveguides), and more particularly, to a method of manufacturing hollow core optical fibers.
Known optical fibers comprise a glass core surrounded by a glass cladding layer. The core is assumed to have a refractive index N1, and the cladding N2. If the relationship N1&gt;N2 is satisfied, light rays entering the fiber are totally internally reflected when the angle of incidence exceeds the critical angle. The light can be transmitted through the fiber to a distant place, being entrapped within the core, enabling transmission with low loss.
The optical fibers for present lightwave systems are made of silica glass containing dopants such as GeO.sub.2 (germanium dioxide). Dopants produce small changes in the refractive index of the glass. By doping the fiber core to a higher refractive index than the surrounding cladding material, a wider variety of materials can be employed to manufacture optical fibers without increasing transmission losses.
In order to minimize transmission losses in optical fibers, many different techniques have been proposed. Transmission losses are due to intrinsic glass scattering and absorption impurity absorption, cladding loss, core/cladding imperfections and geometry loss. Impurity absorption losses occur because transition metal ions and OH groups in the glass absorb light. Scattering loss is due to imperfections in the fiber core, such as bubbles, microcracks and debris as well as composition and density fluctuations. Cladding losses are caused by imperfections at the core-cladding interface. Geometry loss occurs because of bends in the fiber and is an inverse function of the numerical aperture.
U.S. Pat. No. 4,163,654 to Krohn et al. (incorporated by reference) describes various methods for manufacturing optical fibers, including the double crucible process and the chemical vapor disposition process. These proceses generally attempt to overcome some of the problems inherent in fiber manufacturing methods which severely limit the selection of glass compositions used in the manufacturing of optical fibers. U.S. Pat. No. 4,372,767 to Maklad (incorporated by reference) discloses a process for manufacturing low loss optical fibers which overcomes or at least minimizes the above-mentioned transmission losses and is well suited to the production of a low loss optical fiber with an improved index profile.
The CO.sub.2 gas laser emits light in the infrared band region of the spectrum and is widely used for laser machining due to its high efficiency and relatively low cost. Hidaka, et al. disclosed in U.S. Pat. No. 4,453,803 that hollow core optical fibers comprising GeO.sub.2.ZnO.K.sub.2 O cladding compositions can efficiently transmit light in the infrared band and thus can be used with a CO.sub.2 laser with minimal transmission loss. The air core (with refractive index roughly equal to one) is surrounded by glass having a refractive index less than one, thus constituting a true optical waveguide. Transmission loss can be minimized and the fiber tailored to the particular wavelength of light to be used by judiciously controlling the amounts of the additive and the stabilizing components to be incorporated into the glass.
The hollow core fiber has many advantages:
The optical fiber has excellent endurance with respect to high energy beam transmissions due to its hollow core and it displays high mechanical, thermal and chemical stability. PA1 Since the fiber is made from a stable glass oxide, it cannot undergo further degradation of quality and will not undergo loss due to deliquescence and entry of impurities. PA1 Due to its hollow core, the fiber can be subjected to forced air cooling by the passage of cooling air through the hollow core.
Most glass compositions are mechanically weaker and more susceptible to moisture attack than fused silica. This drawback makes the use of other glass compositions for hollow core fiber manufacture less suitable for industrial and medical applications where the flexibility of the fiber is the major advantage. Hidaka (op. cit.) has disclosed that pure GeO.sub.2 glass is such that a hollow core optical fiber made of this material gives the lowest possible transmission loss at the wavenumber of 765 cm.sup.-1. The CO.sub.2 laser beam transmits at a wavelength of 940 cm.sup.-1. Wavenumbers can be adjusted by adding alkali metal oxides (e.g., K.sub.2 O) to pure GeO.sub.2, but when added alone beyond a certain level, the glass assumes a deliquescence, which deprives it of its utility. Therefore, a third component, another oxide (ZnO, BaO or ZrO.sub.2), is added to divest the glass of its deliquescence. The resultant hollow core optical fiber can now transmit light of the middle infrared band, having a wavenumber optimum of 975 cm.sup.-1 to 900 cm.sup.-1 with transmission loss of not more than 0.2 db/M at 940 cm.sup.-1.
To apply this knowledge for the manufacture of hollow core optical fibers, a new process is disclosed wherein the inside of a high silica fiber is coated during the drawings process with desired glass cladding compositions necessary for infrared light guidance, providing a low loss, hollow core optical fiber for laser transmissions.